1. Field of the Invention
This invention relates to attitude control for pitch momentum biased satellites in low inclination orbits, and more particularly, to control of the roll and yaw axes by automatic magnetic torquing in a closed loop system.
2. Description of the Prior Art
A stabilized orbiting satellite requires a means for changing its attitude when it has deviated from its desired orientation or position relative to its orbit.
Magnetic torquing of dual-spin stabilized satellites is known. Such magnetic torquing systems use a magnetic field produced by torquers such as current carrying coils or electromagnets to interact with the magnetic field of the earth to develop a reaction torque. This reaction torque causes the reference axis of the satellite to be repositioned an amount proportional to the torquing time and flux magnitude as is well known in the art.
One closed loop system for controlling the attitude of a satellite having a single inertia wheel mounted on an axis coinciding with the satellite's pitch axis is described in U.S. Pat. No. 3,429,524 issued Feb. 25, 1969 to Westinghouse Electric Corporation, based on the invention of Arthur Buckingham and Thomas Haney entitled "Attitude Control System." The system described therein achieves roll and yaw control by an electromagnetic actuation system which comprises three mutually perpendicular magnetic torquing coils in combination with a three axis magnetometer and both roll and yaw error sensing means. The three axis magnetometer is used to measure the components of the earth's magnetic field in the principal axes of the vehicle. If a roll angle error is sensed by the roll attitude sensing means, an IR horizon sensor, a corrective torque is generated about the yaw axis by energizing the current carrying coils on the pitch and roll axes. Similarly if a yaw angle error is sensed by the yaw attitude sensing means, a corrective torque is generated about the roll axis by energizing the current carrying coils on the pitch and yaw axes. In both instances, on-board computer units, one for each of the satellite's three axes, operate under the conjoint action of the IR sensor, the yaw angle sensor, and the three axis magnetometer to cause the generation of currents which develop the magnetic fields of the torquing coils.
Another closed loop system for controlling roll and yaw error in an orbiting pitch momentum biased satellite is described in U.S. Pat. No. 3,834,653 issued Sept. 10, 1974, to RCA Corporation, based on the invention of Harold Perkel entitled "Closed Loop Roll and Yaw Control and Satellites." The system described therein achieves control by using the sensed errors in roll as the only control input parameter of the closed-loop system. By taking advantage of the gyroscopic properties of the satellite the system obviates the need for a yaw sensor and for an independently stimulated corrective torque about the roll axis in response to the yaw sensor. A magnetic dipole oriented along the roll axis for a satellite in a synchronous or low inclination orbit interacts with the primary magnetic field which is perpendicular to the orbit plane to produce a magnetic control torque about the yaw axis when the roll attitude exceeds a predetermined threshold level. Attitude sensors in combination with electronic logic energize the dipole with currents of appropriate polarity and magnitude to effect the necessary torque to correct the satellite's attitude. Because of gyroscopic cross-coupling between the roll and yaw axes, the control torque causes procession about the roll axis which cancels out the roll errors. Thus, the magnetic torquer directly reduces the roll error and by the gyroscopic cross-coupling indirectly controls the yaw error.
As is well known in the art for such an earth oriented spacecraft, yaw errors are not sensible by earth sensors. Further, roll and yaw errors interchange sinusoidally on approximately a quarter orbit basis throughout the orbit for such a momentum biased spacecraft because of the inertial stiffness of the momentum vector.
In the Perkel system described in the aforesaid patent, a threshold value is typically established for the roll error such that the magnetic dipole is energized only when the roll error exceeds the threshold value. When the roll error is reduced to zero, the torquer is cut off or de-energized. In order to improve the attitude precision in a low inclination orbit, momentum biased spacecraft using the Perkel system, the roll torquing threshold must be lowered. Although lowering of the roll threshold is apparently beneficial to roll precision, the yaw damping which provides the aforementioned indirect control of yaw motion, diminishes. This loss of yaw damping can result in a large yaw build-up and even in a deterioration of roll performance due to the additional disturbance produced by the gyroscopic coupling of the large amplitude yaw errors into roll.
Accordingly, a system for directly minimizing both roll and yaw errors to achieve higher attitude control precision in the absence of a yaw sensing means and an independently stimulated roll torquing means is needed to overcome the deficiencies discussed above of the presently known systems.